1. Field of the Invention
The present invention generally relates to forming patterns on electronic wafers, and more specifically to a new mask technology called SUPREMA (Surface Potential Reflection Electron MAsk) which allows manipulation of reflection characteristics of areas on a mask wafer surface that may be thought of as pixels in a matrix. As a beam illuminates the mask wafer, individual pixels are effectively turned on and off so that the beam will subsequently write the pattern of interest with deep submicron resolution onto a target wafer. In a preferred embodiment providing a programmable mask, writing a new pattern on the mask surface does not require a change of the mask surface itself, but instead changes in software which then controls a voltage of individual pixel-areas on the mask surface.
2. Description of the Related Art
Modern microelectronic manufacturing requires high resolution lithography. As 0.1 μm feature sizes are approached, optical lithography methods are rapidly becoming obsolete. Several large efforts are under way to develop so-called Next Generation Lithography (NGL) methods. These include Extreme Ultra-Violet (EUV), ion beam projection, and electron beam projection lithographies.
The principal efforts in electron beam lithography are “Scalpel”® and “Prevail”®, in which a high energy electron beam passes through a mask. As shown in FIG. 1, in “Scalpel”®, a thin film mask 11 with a metallic scattering layer is used to disperse the electron beam 10 in areas that are not to be exposed on the wafer 12. Wafer 12 has a photoresist layer on its upper surface (not shown in the figure). A contrast aperture 13 in the objective lens backfocal plane removes these scattered electrons from the beam. The thin film mask is supported by an array of struts that periodically interrupt the pattern to be printed.
“Prevail”® uses a stencil mask. Since a stencil mask cannot support free-floating elements, two consecutive masks are required to expose the wafer to a given pattern.
In both schemes, mask and wafer must be scanned together in lockstep, at a fixed demagnification, with the mask stage periodically jumping ahead over the struts. The electron beam sweeps across the mask, exposing the wafer in a manner much like an electron beam sweeping across a CRT monitor. For both “Scalpel” and “Prevail”, the mask technology is complex and expensive, and the resulting masks are extremely fragile.
Like all current lithography methods other than direct e-beam write, a new pattern requires a new mask to be manufactured and introduced into the e-beam projection column. With mask costs easily exceeding $50K, this constitutes a significant cost in the microelectronic manufacturing process. What is needed in the art is a technology that allows mask costs to be reduced while maintaining or even improving resolution and throughput. Prior to the present invention, no such technology has been known.